1. Field of the Invention
The present invention is directed in general to wireless communication systems. In one aspect, the present invention relates to a method and system for controlling the gain and bandwidth properties of an adaptive mixer in a wireless communication device.
2. Related Art
Communication systems are known to support wireless and wire-lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth (BT), advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS) and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device (such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc.) communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over the tuned channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network, via the Internet, and/or via some other wide area network.
Wireless communication devices typically communicate with one another using a radio transceiver (i.e., receiver and transmitter) that may be incorporated in, or coupled to, the wireless communication device. The transmitter typically includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. In direct conversion transmitters/receivers, conversion directly between baseband signals and RF signals is performed. The receiver is typically coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
Wireless communication devices for multimedia audio and video applications generally require a higher data rate than is currently available with devices operating under the 802.11 (a), (b), and (g) standards. Several draft standards (such as the 802.11(n) draft standard) have been proposed to implement faster data rates for wireless devices. Wireless devices operating under these standards will be capable of providing data rates up to 200 Mbps. Since these standards have not been finalized, they will referred to generically as 802.11(x) herein.
Wireless transceivers such as those discussed above in connection with wireless networks employing the various 802.11x standards require very close tolerances to ensure reliable transmission and reception of signals. One of the most important modules in a wireless transceiver is the down converter module wherein incoming radio frequency (RF) signals are converted into an intermediate frequency signal for processing by a baseband unit. A variable gain conversion mixer allows a system level trade-off between linearity and noise figure conversion. Most active mixers have a gain (gm) that is proportional to the load resistance of the mixer. The gain of the mixer, therefore, can be varied by changing the value of the load resistance. Changing the value of the load resistance, however, results in a change of the bandwidth of mixer output filter because the bandwidth is inversely proportional to the load resistance multiplied by the capacitance of the mixer. It is essential in most wireless transceivers that the bandwidth be carefully defined for a particular gain setting. There is a need, therefore, for an improved active filter for use in down conversion modules in wireless interface devices.